(a) Field of the Invention
This invention relates to a method for producing silicon-imide, which is useful as a precursor of silicon nitride.
(b) Prior art description
Sintered silicon nitride is one of the most important material which is able to use as heat resistant structural material for gas turbines and diesel engines, and as high performance material (which is usefull for saving energy or for saving resources) for cutting tool, since it is superior in strength and shock resistance at high temperature, and also in corrosion resistance.
So far, the following four methods have been adopted widely for the production of silicon nitride.
.circle.1 A direct nitridation of silicon by heating metal silicon with catalysis at 1300.degree. C.-1500.degree. C. in nitrogen or ammonia.
.circle.2 A silica reducing method in which is produced by a reduction of silica in nitrogen--the reduction of silica is carried out by heating silica or materials containing silica with carbon in an atmosphere of nitrogen or by the reduction reation with urea.
.circle.3 A gas phase synthesis in which silicon tetra-chloride reacts with ammonia directly at high temperature.
.circle.4 An imide-amide heat decomposition method in which silicon-imide or silicon-amide is obtained by direct ammonolysis of silicon tetra-chloride and then heating the results in a non-oxidative atmosphere to produce silicon nitride.
These processes, however, have many defects as described below.
In the case of the above mentioned method .circle.1 , not only is the reaction time long and heating process complicated, but also the main product is .beta.-type silicon nitride which was rough and contained many impurities.
In the case of the above mentioned method .circle.2 , it is difficult to purify the raw materisls, and in addition, the reaction time is long and produced material has binary system of both .alpha.-type silicon nitride and .beta.-type silicon nitride.
In the case of .circle.3 , since the produced silicon nitride is generally amorphous, not only is a further process for crystallization to obtain .alpha.-type silicon nitride necessary but also it is complicated to deal with the chlorine which is a by-product at the high temperature reaction.
In the case of .circle.4 , although there is the advantage that it is able to produce .alpha.-type silicon nitride of high purity in a good yield, but the flow line of the reaction apparatus is apt to be choked up when silicon-imide or silicon-amide is formed, in addition, there is a defect that it is difficult to control the rapid exothermic reaction.
Furthermore, a method to obtain sintered silicon nitride by heating polysilazane at 800.degree. C.-2000.degree. C., which has been prepared by pyrolyzing organo-polysilazane, was recently proposed (Hajime Saito, Seni Gakkaki Shi, 38 No. 1, 65-72 (1982)). But, this method had the defect of producing silicon carbide and free carbon together with the silicon nitride.
In the method .circle.4 described above, it was a significant problem to prepare easily silicon-imide or silicon-amide with high purity.
As the result of our investigations, we found that by preparing an adduct which is easily formed from the reaction of a halosilane (which is purified easily) with a base, then carrying out the ammonolysis quantitatively on said adduct, a pure silicon-imide and a derived silicon nitride powder of high purity can be obtained, since not only are no catalysts such as Fe and Ca necessary and no residues such as the base remain, but also it was also very easy to remove the chlorine.
Consequently, the object of the present invention is to provide an easy method to prepare silicon-imide (silicon-amide may also be contained), which is useful as a precursor of silicon nitride powder.
The present invention thus comprises a method of synthesizing silicon-imide, as a precursor for preparing silicon nitride powder, wherein an adduct is formed by making the halosilane to react with a base and then reacting the adduct with ammonia.